1. Field of the Invention
The present invention relates to a color thermal printing method and device, and more particularly to an improved color thermal printing method and device for preventing neighboring thermosensitive coloring layers from being colored at the same time.
2. Description of the Related Art
A thermosensitive color recording material has been known (e.g., Japanese Patent Laid-open No. 61-213169) which can directly print a full-color image using a thermal head without using a color ink ribbon. Documents disclosing another type of a thermosensitive color recording material 7 shown in FIG. 1 have been submitted to the Patent Office of Japan (Japanese Patent Application No. 2-89384), although they are not still laid open in public. This thermosensitive color recording material 7 has cyan, magenta, and yellow thermosensitive coloring layers 3, 4, and 5, and a protective layer 6 laminated on a supporting material 2 in this order. In this type of thermosensitive color recording material 7, the heat sensitivity of the outermost yellow thermosensitive coloring layer 5 is highest, and that of the lowermost cyan thermosensitive coloring layer 3 is lowest, as shown in FIG. 2.
The cyan thermosensitive coloring layer 3 contains as its main components an electron-donor type dye precursor and an electron-acceptor type compound, and forms a cyan dye when heated. The magenta thermosensitive coloring layer 4 contains, for example, a diazonium salt compound having a maximum absorption wavelength of 360.+-.20 nm and a coupler which forms a magenta dye when it is thermally reacted with the diazonium salt compound. When an ultraviolet ray of 365 nm for example is applied to the magenta thermosensitive coloring layer 4 after thermal printing, the diazonium salt compound is disposed and the coloring ability the magenta thermosensitive coloring layer 4 is lost. The yellow thermosensitive coloring layer 5 contains, for example, a diazonium salt compound having a maximum absorption wavelength of 420.+-.20 nm and a coupler which forms a yellow dye when it is thermally reacted with the diazonium salt compound. When a near ultraviolet ray of 420 nm for example is applied to the yellow thermosensitive coloring layer 4, it is optically fixed and the coloring ability of the yellow thermosensitive coloring layer 4 is lost.
When recording a full-color image on the above-described thermosensitive color recording material 7, a thermal head having a plurality of heating elements arranged in a line is used. First, the yellow thermosensitive coloring layer 5, or the first layer that is disposed on the outermost of the coloring layers, is thermally recorded, while the thermal head is moved relative to the thermosensitive color recording material 7. During the thermal recording, each heating element of the thermal head is applied with a bias pulse having a relatively large width for heating the thermosensitive color recording material 7 up to near the coloring temperature. Then a number of image pulses having a smaller width for changing the power-on time depending upon the pixel optical density of an original image and forming color pixels having a desired optical density is applied. This method of driving heating elements is described, for example, in Japanese Patent Laid-open Publication No. 3-221468. After thermally recording a yellow image, a near ultraviolet ray of 420 nm is applied to optically fix the yellow image. Next, the magenta thermosensitive coloring layer 4, or the second layer, is thermally recorded by using a higher heat energy than that applied for the yellow thermosensitive coloring layer 5. Thereafter, the magenta image is optically fixed by being exposed to an ultraviolet ray of 365 nm. Lastly, the cyan thermosensitive coloring layer 3, or the third layer, is thermally recorded by using a heat energy higher than the heat energy used for the thermosensitive color layers 4 and 5.
The thermosensitive color recording material 7 has intermediate layers formed between the thermosensitive coloring layers, although an illustration of the intermediate layers is omitted in FIG. 1. If the intermediate layers are made too thick, the heat sensitivity lowers to degree which poses a problem in practical use. However, by setting a proper thickness of the intermediate layers, the overlap between coloring characteristic curves can be avoided. Such a thermosensitive color recording material having no overlap of the characteristic curves has been proposed, for example, in Japanese Patent Application No. 2-134303. This thermosensitive color recording material is thermally recorded in the following manner. First, the yellow thermosensitive coloring layer (third thermosensitive coloring layer) is heated by a thermal head so that the heat only allows the yellow thermosensitive coloring layer to develop color. As a result, the diazonium salt compound contained in the layer reacts with the coupler and a yellow dye is formed. After the third thermosensitive coloring layer is thermally recorded and optically fixed, the magenta thermosensitive coloring layer (second thermosensitive coloring layer) is heated by the thermal head, so that the heat only allows the magenta thermosensitive coloring layer to develop color and the cyan thermosensitive coloring layer (first thermosensitive coloring layer) is not allowed to develop color. After the second thermosensitive coloring layer is optically fixed, the first thermosensitive coloring layer is heated to develop cyan color. The half tone image for yellow, magenta, and cyan can be independently recorded without color mixture by driving the thermal head under the following conditions: Thermal head: 0.5 W/dot printing energy (manufactured by Kyocera
Electronics); PA0 Pixel density: 8 pixels/mm; PA0 Yellow: 0.4 to 2.0 ms; PA0 Magenta: 2.4 to 4.0 ms; PA0 Cyan: 4.4 to 6.0 ms.
Thermal head driving pulse: having a constant voltage and a power-on time changing by 0.2 ms pitch depending on the tone level.
As seen from the characteristic curves of FIG. 2, if the heat sensitivity of each thermosensitive coloring layer of the thermosensitive color recording material is raised to reduce the recording heat energy at the area EA, the high density area of the yellow thermosensitive coloring layer 5 overlaps with the low density area of the magenta thermosensitive coloring layer 4. Therefore, when a high density image for yellow is recorded, the magenta thermosensitive coloring layer 4 develops color by the heat energy applied for coloring the yellow image to cause color mixture, so that the original color hue is unable to be produced. Furthermore, at the area EB, the high density area of the magenta thermosensitive coloring layer 4 overlaps with the low density area of the cyan thermosensitive coloring layer 3, which poses the same problem as discussed above. In view of this, for the thermal recording of the yellow thermosensitive coloring layer 5 and the magenta thermosensitive coloring layer 4, the heat energy which does not allow the layer under each thermosensitive coloring layer to develop color, has been used conventionally. For this reason, with a conventional thermal color printing method, a high density image cannot be recorded in some cases.